Controlling the Phase and Amplitude of Plasmon Sources at a Subwavelength Scale

Abstract

We present a new class of nanoscale plasmonic sources based on subwavelength dielectric cavities embedded in a metal slab. Exploiting the strong dispersion near the Fabry-Perot resonance in such a resonator, we control the phase and the amplitude of the generated plasmons at the subwavelength scale. As an example, we present a subwavelength unidirectional plasmonic antenna utilizing interference between two plasmonic cavity sources with matched phase and amplitude.Surface plasmons polaritons (SPPs) are collective excitations of electrons coupled to an electromagnetic field at the interface between a dielectric and a metal.1 Their evanescent nature along the normal to the metal/dielectric interface allows subwavelength confinement that can be significantly smaller than diffraction limited optical waves in bulk media. SPPs, therefore, are ideal candidates for the construction of subwavelength optical devices. In the past few years, SPPs have also been identified to be the major physical mechanism involved in the extraordinary transmission of light (ETL) through metal films with subwavelength holes which was first reported by Ebbesen et al. 2,4 Subsequent studies have been made on uniform periodic arrays of holes, slits, and more complex shapes fabricated in metallic films. Those studies demonstrate that one can take advantage of the interference between plasmons generated on metal films by uniform periodic sources (e.g., slits or holes in metal films) to, for instance, focus or disperse light, 5,6 or realize ETL. Naturally, for flexible control and manipulation of light by such metal films it is necessary to evolve beyond the uniform periodic sources in refs 2-7 and introduce the rich possibilities afforded by nonuniform source films. When light enters a subwavelength dielectric structure in a metallic film, a significant fraction, if not all, of the light propagates through the film as surface plasmons that are confined at the metal/dielectric interfaces. For example, transverse magnetic (TM) polarized light impinging on a silver film containing air gaps gives rise to waves known as gap plasmons (GPs), whose properties are closely related to the dimensions of the gaps: the smaller the gap width, the larger the wavenumber of the GP. 11,12 This way, one can design sources of plasmons with arbitrary phase by adjusting the width of the gap. Similarly, one can fill the air gaps with different dielectrics hence inducing an optical path length between the generated plasmons. 8,10 Those two approaches are quite inflexible and practically hard to fabricate.In order to realize surface plasmon sources with chosen phase and amplitude, our approach utilizes the gap plasmon dispersion relation along with Fabry-Perot (FP) resonances in a cavity. 13,14 Air gaps in metal films typically display lowefficiency FP resonance 13 that, as we explain later, can be increased by introducing highly reflective "mirrors" on both sides of the metal film. Sharper resonance results in longer propagation of the GPs in the gaps due to con...

Ultra-broadband unidirectional launching of surface plasmon polaritons by a double-slit structure beyond the diffraction limit

“…By adopting the chirped plasmonic gratings with a lateral dimension of about 17 μm 23 or 4 μm, 24 the bandwidths of the unidirectional SPP source reached 190 nm 23 and 210 nm. 24 With a pair of phased nanoslits, 25 the broadband unidirectional SPPs with a bandwidth of about 300 nm were numerically predicted, while the generated SPPs from all of these SPP launchers propagated bulkily along the metal surface, [3][4][5][6][7][8][9][10][12][13][14][15][16][17][18][19][20][21][22][23][24][25] leading to diverging SPP sources or approximate plane-wave SPP sources on the metal surface.…”

“…Although some of these unidirectional SPP launchers possessed very small lateral dimensions ( parallel to the SPP propagation direction along the metal surface), which can even be downscaled to subwavelength, [12][13][14]22,23 nearly all of the unidirectional SPP launchers had very large longitudinal dimensions (>10λ, perpendicular to the SPP propagation direction along the metal surface). [3][4][5][6][7][8][9][12][13][14][15][16][17][18][19][20][21][22][23][24][25] This significantly limits the on-chip integration density of the plasmonic devices. Moreover, the generated SPPs propagated bulkily along the metal surface without any confinements.…”

“…Moreover, the generated SPPs propagated bulkily along the metal surface without any confinements. [3][4][5][6][7][8][9][10][12][13][14][15][16][17][18][19][20][21][22][23][24][25] This makes it hard to directly couple the generated SPPs to subwavelength plasmonic waveguides. What is worse, the bulky propagation of the SPPs would bring large crosstalk between different plasmonic devices in high-integration plasmonic circuits.…”

“…[20][21][22][23][24][25] Moreover, its broadband and efficient properties are still maintained when the slit length is reduced to a subwavelength scale (<λ/10), which is about one hundred times smaller than that in the previous unidirectional SPP launchers. [3][4][5][6][7][8][9][12][13][14][15][16][17][18][19][20][21][22][23][24][25] This significantly shrinks the occupied area of the SPP launcher on the metal surface to be smaller than λ 2 /10. To avoid the bulky propagation of the SPPs along the metal surface, such a robust unidirectional SPP launcher beyond the diffraction limit is directly coupled to a subwavelength plasmonic waveguide.…”

Ultrawide‐Band Unidirectional Surface Plasmon Polariton Launchers

“…[6,7] The other approach is to use asymmetric configuration to help control the launching direction of SPPs based on linear interference effect. [8,9] However, only a single operating wavelength was achieved experimentally due to the difficulty in precisely controlling the optical phase difference within a broad frequency range. [10,11] For example, In 2007, Tejeira et al reported an unidirectional SPP launching with a launching efficiency ratio of 2, operating at the wavelength of 800 nm.…”